1. Field of the Invention
The present invention relates to a low-noise differential amplifier required to have a high dynamic range at a wideband.
2. Description of the Related Art
A non-patent literature (K. van Hartingsveldt, M. H. L. Kouwenhoven, C. J. M. Verhoeven, “HF Low Noise Amplifiers with Integrated Transformer Feedback”, ISCAS 2002, vol. 2, pp. II-815 to II-818, May 2002) (hereinafter, non-patent literature 1) discloses a low-noise amplification circuit having a duplex negative feedback network comprising a transformer and a resistor (Transformer Feedback Degenerated Low Noise Amplifier, hereinafter, TFD-LNA). The TFD-LNA is a good circuit which can achieve all of low noise figure, stable gain and good input impedance matching at a wideband.
A differential amplifier using the TFD-LNA is, however, not a well known art.
Accordingly, a differential amplifier which maintains a low noise figure, a stable gain, and a good input impedance matching at a wideband is expected using the TFD-LNA of non-patent literature 1 as right side and left side amplifiers. Let us now think about a differential amplifier to which a phase compensation network is added in order to cause the amplifier to have a high dynamic range at a wideband with a sufficient stability margin. Then, a differential amplifier shown in FIG. 12 (this amplifier is hereinafter called basic type TFD differential amplifier) can be thought out.
FIG. 12 shows the basic type TFD differential amplifier having a pair of TFD-LNAs as right and left amplifiers.
A basic type TFD differential amplifier 10 comprises symmetrical right and left amplifiers having a common circuit constant. The left amplifier of the basic type TFD differential amplifier 10 includes transistors 24, 27, and 31. The right amplifier of the basic type TFD differential amplifier includes transistors 54, 57, and 61. The right and left amplifiers of the basic type TFD differential amplifier 10 individually have input and output terminals. A node between a resistor 39 and the primary winding of a transformer 23 serves as the input terminal of the left amplifier, while a node between a resistor 69 and the primary winding of a transformer 53 serves as the input terminal of the right amplifier. The emitter of the transistor 31 functions as the output terminal of the left amplifier, while the emitter of the transistor 61 functions as the output terminal of the right amplifier. The differential-signal input terminal of the basic type TFD differential amplifier 10 comprises the input terminal of the left amplifier and the input terminal of the right amplifier. The emitter of the transistor 31 and that of the transistor 61 are a pair of right and left output terminals of the basic type TFD differential amplifier 10.
According to the basic type TFD differential amplifier 10, a signal source having an output impedance R of 50Ω is connected to the hot side of a primary winding of a balun transformer 12. The cold side of the primary winding of the balun transformer 12 is grounded. Both ends of a secondary winding of the balun transformer 12 are respectively connected to the input terminals of the right and left amplifiers of the basic type TFD differential amplifier 10 through coupling capacitors 21, 22. The balun transformer 12 converts a single-ended input signal into a differential signal. The turn ratio between the primary winding of the balun transformer 12 and the secondary winding thereof is, for example, 1:1.
The hot side of the primary winding of the transformer 23 is connected to the input terminal of the left amplifier. A commercially available transformer having a turn ratio of 1:2 is used as the transformer 23.
The cold side of the primary winding of the transformer 23 is connected to the base of the NPN type transistor (hereinafter, simply called transistor) 24. The base of the transistor 24 is also connected to the positive electrode of a biasing power source 25 through a choke coil 26.
The collector of the transistor 24 is connected to the emitter of the transistor 27. The base of the transistor 27 is connected to the positive electrode of a biasing power source 29 through a phase compensation resistor 28. The resistor 28 works together with a capacitor 38 to be discussed later, and constitutes a phase compensation circuit for performing phase compensation on the left amplifier of the basic type TFD differential amplifier 10. The negative electrode of the biasing power source 29 is grounded.
The transistor 24 and the transistor 27 are subjected to cascode connection with each other, and constitute a cascode amplifier having a resistor 30 as a load. The collector of the transistor 27 is connected to one electrode of the resistor 30 which functions as the load device of the cascode amplifier. A direct-current-power-source voltage Vd1 is applied to the other electrode of the resistor 30.
A node between the resistor 30 and the collector of the transistor 27 serves as an output node for outputting an amplified output voltage signal of the cascode amplifier. The node is connected to the base of the transistor 31, i.e., the input terminal of an emitter follower. The transistor 31 and a constant-current source 35 constitute the emitter follower, and works as the output buffer of the left amplifier of the basic type TFD differential amplifier 10. The direct-current-power-source voltage Vd1 is applied to the collector of the transistor 31. The emitter of the transistor 31 is connected to one electrode of the coupling capacitor 32.
The left output terminal of the basic type TFD differential amplifier 10, i.e., the emitter of the transistor 31 is connected to the cold side of the secondary winding of the transformer 23 through a coupling capacitor 34. An output voltage signal applied to the secondary winding of the transformer 23 is transmitted to the primary winding of the transformer 23 by electromagnetic coupling, and is series-mixed with an input signal. This constitutes one negative feedback network in the basic type TFD differential amplifier 10. The emitter of the transistor 31 is connected to a constant-current source 35 for providing an operating current of the emitter follower.
The emitter of the transistor 31 is further connected to one electrode of a coupling capacitor 36, one electrode of a phase compensation capacitor 37, and one electrode of a phase compensation capacitor 38.
A resistor 39 and a coupling capacitor 36 are connected in series between the left output terminal of the basic type TFD differential amplifier 10 and the hot side of the primary winding of the transformer 23, i.e., the left signal input terminal of the basic type TFD differential amplifier 10. The resistor 39 shunt-mixes a voltage-sampled output signal with an input signal. This constitutes one negative feedback network in the basic type TFD differential amplifier 10.
The capacitor 37 and a resistor 40 constitute a phase compensation network for performing phase compensation on the left amplifier of the basic type TFD differential amplifier 10.
One electrode of a capacitor 22 is connected to the hot side of a primary winding of a transformer 53. A commercially available transformer having a turn ratio of 1:2 is used as the transformer 53.
The cold side of the primary winding of the transformer 53 is connected to the base of a transistor 54. The base of the transistor 54 is further connected to the positive electrode of a biasing power source 55 through a choke coil 56.
The collector of the transistor 54 is connected to the emitter of a transistor 57. The base of the transistor 57 is connected to the positive electrode of a biasing power source 59 through a resistor 58. The resistor 58 works together with a capacitor 68 to be discussed later, and constitutes a phase compensation network for performing phase compensation on the right amplifier of the basic type TFD differential amplifier 10. The negative electrode of the biasing power source 59 is grounded.
The transistors 54, 57 are subjected to cascode connection with each other, and constitute a cascode amplifier having a resistor 60 as a load. The collector of the transistor 57 is connected to one electrode of the resistor 60 which serves as the load device of the cascode amplifier. The direct-current-power-source voltage Vd1 is applied to the other electrode of the resistor 60.
A node between the resistor 60 and the collector of the transistor 57 serves as an output node for outputting an amplified output voltage signal of the cascode amplifier. The node is connected to the base of a transistor 61, i.e., the input terminal of an emitter follower. The transistor 61 and a constant-current source 65 constitute the emitter follower, and works as the output buffer of the right amplifier of the basic type TFD differential amplifier 10. The direct-current-power-source voltage Vd1 is applied to the collector of the transistor 61. The emitter of the transistor 61 is connected to one electrode of a coupling capacitor 62.
The right output terminal of the basic type TFD differential amplifier 10, i.e., the emitter of the transistor 61 is connected to the cold side of the secondary winding of the transformer 53 through a coupling capacitor 64. An output voltage signal applied to the secondary winding of the transformer 53 is transmitted to the primary winding of the transformer 53 by electromagnetic coupling, and is series-mixed with an input signal. This constitutes one negative feedback network in the basic type TFD differential amplifier 10.
The emitter of the transistor 61 is connected to a constant current source 65 for providing an operating current of the emitter follower.
The emitter of the transistor 61 is further connected to one electrode of a coupling capacitor 66, one electrode of a phase compensation capacitor 67, and one electrode of a phase compensation capacitor 68.
A resistor 69 and the coupling capacitor 66 are connected in series between the right output terminal of the basic type TFD differential amplifier 10 and the hot side of the primary winding of the transformer 53, i.e., the right signal input terminal of the basic type TFD differential amplifier 10, and shunt-mix a voltage-sampled output signal with an input signal. This constitutes one negative feedback network of the basic type TFD differential amplifier 10.
The capacitor 67 and a resistor 70 constitute a phase compensation network for performing phase compensation on the right amplifier of the basic type TFD differential amplifier 10.
The emitters of the transistors 24, 54 are connected to the collector of a transistor 71. The base of the transistor 71 is connected to the base of a transistor 72 and the collector thereof, and the transistors 71, 72 constitute a current mirror circuit.
The collector of the transistor 72 is connected to a constant-current source 73. The emitter of the transistor 71 is grounded through a resistor 74. The emitter of the transistor 72 is grounded through a resistor 75. The collector current of the transistor 71 is controlled in such a manner as to be always constant by a constant-current source 73. Accordingly, the right and left amplifiers of the basic type TFD differential amplifier 10 operate in such a way that the right and left output signals become a balanced signal having an always-constant sum.
The emitter of the transistor 31 and the emitter of the transistor 61 are a pair of differential output terminals of the basic type TFD differential amplifier 10. Those output terminals are connected to both ends of a primary winding of a balun transformer 80 through coupling capacitors 32, 62, respectively. The hot side of a secondary winding of the balun transformer 80 is connected to a load 81 of, for example, 5 kΩ. The balun transformer 80 converts the differential amplified output signal of the basic type TFD differential amplifier 10 into a single-ended signal. The turn ratio of the balun transformer 80 is, for example, 1:1.
Here, an explanation will be given of a result of simulating the characteristic of the basic type TFD differential amplifier 10 in a case where the balun transformers 12, 80 are ideal transformers having a turn ratio of 1:1.
FIGS. 13A to 13C show respective simulation results for a noise figure (NF), a reflection coefficient (S11) and a transmission coefficient (S21) of the basic type TFD differential amplifier 10 shown in FIG. 12.
It becomes clear from the simulation result that the basic type TFD differential amplifier 10 realizes all of sufficient noise figure characteristic, sufficient input impedance characteristic, and stable voltage gain of about 7 dB at bands up to about 200 MHz. In a case where an actual transformer is used as the balun transformer 12 which converts a single-phase input from an antenna into a differential input, the NF value in a band where the amplifier can be operated within normal specifications deteriorates about 0.5 to 1 dB from the foregoing simulation result.
FIG. 14 shows a result of simulating the third order input intercept point (IIP3) characteristic of the basic type TFD differential amplifier 10. The horizontal axis represents a frequency (MHz), while the vertical axis represents an IIP3 (dBm).
In the measurement simulation of the IIP3 characteristic, two tone signals each having −50 dBm power at a frequency differing from a measurement frequency by ±10 kHz are used as input signals. According to the simulation, it becomes clear that the IIP3 greater than or equal to +45 dBm is maintained up to 100 MHz, and the high IIP3 greater than or equal to +25 dBm is maintained across a wideband up to 300 MHz.
The basic type TFD differential amplifier 10 has symmetrical circuit forms. Accordingly, in an ideal condition, no even-order distortion is present in the output signal of the basic type TFD differential amplifier 10. Moreover, as shown in FIGS. 13A to 13C and FIG. 14, the basic type TFD differential amplifier 10 having combined TFD-LNAs can realize a high dynamic range at a wide band.
However, when the basic type TFD differential amplifier 10 which can perform differential imputing/outputting is constituted using a pair of TFD-LNAs disclosed in non-patent literature 1, one transformer is required for the negative feedback network of the individual right or left amplifier, a total of two high-frequency transformers 23, 53 are required. Moreover, in a case where the basic type TFD differential amplifier 10 is embedded with a radio communication device, an input signal from an antenna is given as a single-phase signal. Accordingly, as shown in FIG. 12, the high-frequency balun transformer 12 is generally provided ahead of the input stage of the differential amplifier to convert the single-phase signal into the differential signal. For the differential amplifier using a pair of low-noise amplification circuits disclosed in non-patent literature 1, a total of three high-frequency transformers including the balun transformer 12 are required.
The high-frequency transformers 23, 53 and the balun transformer 12 are relatively expensive parts, and have a large occupying area on a substrate or a printed circuit board. Accordingly, using three high-frequency transformers is not desirable because of a lack of cost competence.
Moreover, an actual transformer generates a thermal noise which cannot be ignored. Accordingly, when the number of transformer used increases, the noise figure of the basic type TFD differential amplifier 10 deteriorates. Therefore, it is desirable to reduce the number of transformers to be used in order to improve the noise characteristic of the basic type TFD differential amplifier 10.
The basic type TFD differential amplifier 10 shown in FIG. 12 is an example which uses cascode amplifiers. In a case where a basic type TFD differential amplifier comprising another type of amplifiers other than the cascode type is to be designed, three transformers are still required.